Generally, a motor is operated at a high speed and a low torsion force. In other words, it is difficult to drive a large-sized load. Consequently, for allowing the motor to drive a heavy object, a speed reducer is used for reducing the rotating speed of the motor and increasing the torsion force.
Conventionally, the speed reducers are classified into several types, including rotary vector (RV) reducers and harmonic drive reducers. For example, the RV-E series reducer is a two-stage speed reducer produced by Nabtesco. The RV-E series reducer comprises a first speed reduction stage with a spur gear and a second speed reduction stage with an epicyclic gear. The gears of the first speed reduction stage and the second speed reduction stage are made of metallic material. The RV-E series reducer is a two-stage reduction design to reduce vibration and inertia while increasing ratio capabilities. The RV-E series reducer provides high-end performance in highly rigid and high reduction ratio configurations, and the rolling contact elements of the RV-E series reducer have high efficiency and long life. However, since the RV-E series reducer has larger volume and weight and has many components, the cost of the RV-E series reducer is high.
The harmonic drive reducer comprises a wave generator, a flex element (e.g., a flex spline) and a rigid gear. The elastic deformation of the flexible element can be controlled to result in a pushing action so as to transmit motion and power of mechanical transmission. The harmonic drive reducer has smaller size, lighter weight and higher precision when compared with the RV reducer. However, since the rigidity of the flex element of the harmonic drive reducer is lower than the metallic element, the harmonic drive reducer is unable to withstand high impact and has a problem of causing teeth difference friction. In other words, the use life of the harmonic drive reducer is shorter. Moreover, since the input speed of the harmonic drive reducer is not high, the reduction ratio of the harmonic drive reducer is lower.
Therefore, there is a need of providing a speed reducer with the characteristics of a RV reducer and a harmonic drive reducer in order to the overcome the above drawbacks.